The present invention relates to a piston. In more detail, the present invention relates to a piston for use in a cylinder of an internal combustion engine, wherein the shape of the piston reduces the soot emissions of the internal combustion engine.
Soot particles, or particulates, are a product which, during combustion, can be formed and subsequently oxidized into carbon dioxide, CO2.
Combustion with fuel, i.e. rich, fuel/air mixture with poor mixing at high temperature produces high soot formation. If the formed soot particles can be brought together with oxidizing substances such as e.g. oxygen atoms, oxygen molecules, hydroxide at sufficiently high temperature for a good oxidation rate, a greater part of the soot particles can be oxidized. In a diesel engine, the oxidation process is considered to be in the same order of magnitude as the formation, which means that net soot production is the difference between formed quantity of soot and oxidized quantity of soot. The net emission of soot can therefore be influenced firstly by reducing the formation of soot and secondly by increasing the oxidation of soot. Carbon monoxide emissions and hydrocarbon emissions are normally very low from a diesel engine. However, the percentages can rise if unburnt fuel ends up in relatively cool regions. Such regions are, for example, zones with intense cooling located close to the cylinder wall.
A combustion process in which the fuel is injected directly into the cylinder and is ignited by increased temperature and pressure in the cylinder is generally referred to as the diesel process. When the fuel is ignited in the cylinder, combustion gases present in the cylinder undergo turbulent mixing with the burning fuel, so that a mixture-controlled diffusion flame is formed. The combustion of the fuel/gas mixture in the cylinder gives rise to heat generation, which causes the gas in the cylinder to expand and which hence causes the piston to move in the cylinder. Depending on a number of parameters, such as the injection pressure of the fuel, the quantity of exhaust gases re-circulated to the cylinder, the time of injection of the fuel and the turbulence prevailing in the cylinder, different efficiency and engine emission values are obtained.
An approach to solve the problem of reducing soot emissions is disclosed in WO 2009/058 055, which describes an internal combustion engine. The internal combustion engine in WO 2009/058 055 comprises a piston having protrusions mutually spaced apart from each other in a circumferential direction of the piston. These protrusions are configured to receive a flame of burned fuel. When the flame impinges a wall between a pair of protrusions, the flame will be directed towards the protrusions which will direct the flame back towards the centre of the cylinder. Hereby, the amount of soot can be reduced by means of the flame recirculation.
Although the piston described in WO 2009/058 055 reduces soot emissions, it is still in need of further improvements such as e.g. providing an increased degree of combustion efficiency for the internal combustion engine in which it is positioned.
It is desirable to provide a piston having such shape and functions that, when in use, further reduces the soot emissions from the internal combustion engine.
According to a first aspect of the present invention there is provided a piston for a cylinder of a combustion engine, the piston comprising: a piston crown comprising a piston bowl formed by an upwardly facing cavity; the piston bowl including a floor section with a central apex, an inner section extending downwardly from the apex, and a side section, the side section connecting the inner section with an upper surface of the piston crown, wherein the side section is formed with mutually spaced apart ridges protruding towards the apex, and wherein the side section having at least one recess at an upper end of the side section connecting the side section with the upper surface of the piston crown.
The present invention is based on the insight that during ignition in a cylinder of an internal combustion engine a flame that impinges the side section below the at least one recess will not only be directed towards the ridges, but will also be directed in an upward direction towards the cylinder liner. By providing a recess in the piston crown, a majority of the part of the flame directed towards the cylinder liner will instead become directed towards the cylinder head and thereafter back towards the centre of the cylinder. An advantage of the invention is, at least, that a reduction of soot emissions is provided for the internal combustion engine in which the piston is positioned, since the combination of a ridge and a recess increases the amount of the flame directed back towards the centre of the cylinder. Also, by providing a recess in accordance with the present invention enables for a reduction of a spray axis angle, i.e. the angle of which the spray is injected into the cylinder, which will be further described below. A reduction of the spray axis angle provides for an improved combustion process of the combustion chamber in the cylinder, since a larger amount of the flame will be directed towards the ridges and thereby be directed towards the centre of the cylinder which reduces the loss of kinetic energy of the flame. The recess will also direct the upwardly portion of the flame towards the centre of the cylinder.
The present invention is directed towards pistons which are configured to be arranged in internal combustion engines with no, or low, swirling effect during operation. The ratio of swirl in an internal combustion engine is a ratio of the tangential velocity of the air spinning around the combustion chamber divided by the engine speed. Accordingly, the swirl ratio is a measure of the tangential motion of air as it enters the engine cylinder from induction ports of the cylinder head. The swirl ratio of the present invention is below 0.7 which is considered to be low swirl.
The wording “upper surface” should be interpreted as the surface of the piston which is, when the piston is positioned in a cylinder of a combustion chamber, faces the cylinder head and hence being the part of the piston located closest to the cylinder head. Similarly, the wording “upwardly facing” should thus be interpreted as facing the cylinder head when the piston is mounted in the cylinder. The wording “upper” and “upwardly” hence correspond to the direction away from an engine crank shaft when the piston is positioned in a cylinder.
According to an example embodiment of the present invention, ridges and recesses may be formed alternatingly in the circumferential direction of the side section.
According to an example embodiment of the present invention, the at least one recess may be positioned at an equal distance from its closest ridges.
Hereby, the ridges are arranged symmetrically in relation to the at least one recess. The flame will thus impinge the side section and be directed not only to the recess, but towards each of the adjacent ridges. The flame will, due to the equal distance between the at least one recess and the respective ridges, reach each of the adjacent ridges at an approximately equal point in time.
According to an example embodiment of the present invention, the side section may have a concave curvilinear shape in cross section. According to an example embodiment of the present invention, the at least one recess may be symmetrically formed in relation to a respective plane extending in an axial and radial direction of the piston crown, the geometric plane crossing the recess at a centre thereof in a circumferential direction.
In a cylinder having no, or low, swirling effect, the flame will impinge the side section substantially perpendicular to the side section. An advantage of having a recess which is symmetrical is thus that the flame may be controlled such that it will be directed towards a centre of the cylinder after being received in the recess.
According to an example embodiment of the present invention, each of the ridges may be symmetrically formed in relation to a respective plane extending in an axial and radial direction of the piston crown, the geometric plane crossing the ridge at a centre thereof in a circumferential direction.
An advantage is that the portion of the flame that is directed towards the ridge will also, as for the portion of the flame directed into the recess, be directed towards a centre of the cylinder when the flame arrives at the ridge.
By controlling the flame such that a majority of the flame, after impinging the side section, is provided into a centre of the cylinder, the combustion process is improved, i.e. the efficiency of the internal combustion engine is increased. Also, and as described above, soot emissions are reduced and the risk of contaminating e.g. the piston rings will be reduced since the flame is provided towards the centre of the cylinder away from the piston rings. Further, controlling the flame in accordance with the above, the combustion process may be executed more optimal, which is beneficial in relation to combustion executed “too late” in a cycle, as this reduces the thermodynamic efficiency.
According to an example embodiment of the present invention, at least one of the ridges may extend into at least a portion of the inner section.
Hereby, the ridges may be designed to be large enough to guide the flame towards the centre of the cylinder.
According to an example embodiment of the present invention, at least one of the ridges may extend from the upper surface towards the inner section.
Accordingly, there is flexibility when designing the ridges, which may be designed differently depending on e.g. the size and configuration of the piston and/or cylinder in which the piston is configured to be provided into.
According to an example embodiment of the present invention, each of the recesses may have a horizontal seat section and a vertical wall section, wherein the horizontal seat section is connected to the vertical wall section by means of a radius. According to an example embodiment, the vertical wall section may be approximately 15-30% of the depth of the piston and the horizontal seat section may have a length in the range from the length of the vertical wall section up to twice the length of the vertical wall section.
According to a second aspect of the present invention there is provided an internal combustion engine comprising a cylinder and a piston according to any one of the above described embodiments.
Effects and features of this second aspect are largely analogous to those described above in relation to the first aspect of the present invention.
According to an embodiment of the present invention, the internal combustion engine is a low-swirl internal combustion engine.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.